Adjustable inductance



May 20, 1958 L. HAMMoND 2,835,876

- ADJUSTABLE INDUCTANCE y Filed Aug. 18, 1950 United States Patent ADJUSTABLE INDUCTANCE Laurens Hammond, Chicago, Ill., assignor to Hammond Organ Company, a corporation of Delaware Application August 18, 1950, Serial No. 180,289

1 Claim. (Cl. 336-134) My invention relates generally to adjustable inductance elements, particularly elements of this type adapted for use in audio-frequency circuits in which the value of the inductance must be very accurately adjustable.

In the manufacture of audio-frequency oscillators and similar audio-frequency equipment it is frequently desirable to provide a cheap, simple, and effective inductance device which may be easily varied or adjusted in small increments so as to enable the circuit in which the inductance is used to be very accurately adjusted to a certain resonant frequency. This is particularly the case in the manufacture of electrical musical instruments in which the tone signal frequencies are generated by means of vacuum tube oscillators employing inductancecapacitance tuned circuits for determining the frequency of oscillation. Since there may be many such oscillators employed in a musical instrument for providing the large number of frequencies requiredin the rendition ol music it is important that the cost of each oscillator be reduced to the minimum. Furthermore, in a musical instrument the oscillators must be capable of being tuned very accurately to the desired pitch or frequency and this requirement can most easily be met by providing an inductance element, the inductive value of which may be changed in extremely small increments, because, in general, each oscillator must be tuned to its correct pitch with an accuracy of about one part in 300, or approximately 0.3%.

It is also highly desirable that any inductances used in the tuning circuits of the oscillators of a musical instrument be capable of being adjusted quickly and easily not only during the manufacture of the oscillators, but should also be so simple that adjustments may be made by the users of such instruments.

It is therefore the primary object of my invention to provide an improved inductance element which may be manufactured at low cost, which may easily and quickly be adjusted, within a wide range of values, to a certain value, and in which such adjustment may be easily eected with a high degree of precision.

A further object is to provide an improved inductance element and an improved method of adjusting its effective value.

Other objects will appear from the following description, reference being had to the accompanying drawing in which:

Figure l is a plan View of the improved inductance element shown mounted on a plate;

Figure 2 is an enlarged side elevational view, the mounting plate being shown in section;

Figure 3 is a horizontal sectional view taken on the line 3-3 of Fig. 2, shown to the same scale as Fig. l;

Figures 4 and 5 are plan views indicating the manner in which the value of the inductance element is adjusted; and

Figure 6 is a schematic wiring diagram of an oscillator in which the inductance of the invention may be emplfyed advantageously.

2,835,876` Patented May 20, 1958 The inductance element comprises a coil 10 wound on a suitable hollow form made of sheets 12, 13, and 34, of insulating material, suitably interlocked with end sheets 14, the form in the coil being positioned over center legs 16 of two groups or stacks of generally E-shaped iron laminations 18 and 19. These groups of laminations are composed of identical stampings, each compristhe center leg portion 16, a long leg portion 20, and a short leg portion 22. Alternate pairs of laminations of each stack are reversed so that, as best indicated in Fig. 2, in each group or stack adjacent legs of pairs of laminations will be alternately long and short with the extended portions of the long legs of the two groups 18 and 19 interleaved. The two groups of laminations thus together form a binocular core having greater or less air gap between the ends of the center legs 16 and additional minor air gaps between the ends of the legs 20 of one stack and the legs 22 of the other stack.

All of the laminations of the stack 18 are tightly secured together by a pair of bolts 24 provided with nuts 26, the lower ends of the bolts 24 extending freely through elongated slots 28 formed in a mounting plate 30. The group of laminations 19 are tightly secured together by a pair of bolts 32 which also clamp the insulating sheet 34 in place, the bolts having nuts 36 which are preferably cylindrical in outer contour and extend through locating holes 38 formed in the mounting plate 3i). The sheet 34 forms a terminal strip to which soldering lugs 35 are riveted. coil 18, and a lead wire to a tap on the coil (not shown) are soldered to these lugs 35. The mounting plate 30 is provided with a large rectangular opening 40 through which the lower portion of the coil and its form project.

The coil structure is clamped to the mounting plate 30 by means of a generally U-shaped clamping strap 42. The end portions 50 of the strap 42 are secured to the plate by screws 44 threaded in pierced and tapped bosses 46 in the mounting plate 30. A spring lock washer 48 and a plain washer 49 are positioned beneath the head of each of the screws 44, and the overlapping portions of the two groups of laminations 18 and 19 are therefore resiliently pressed between the end portions 50 of the clamping strap 42 and spacing washers 52 which are located between the mounting plate 30 and the lowermost laminations. The force supplied by the spring washers thusfrictionally clamps the two groups of laminations together and holds them against movement relative to each other or relative to the mounting plate 30.

By virtue of this construction of the core laminations and the manner in which they are clamped in place, it is possible very easily and quickly to adjust the position of one group of laminations relative to the other to change the air gap and thus to alter the etective value of the inductance.

The manner in which this adjustment may be made is illustrated in Figs. 4 and 5. In order to decrease the inductance value of the coil the two screws 44 are first unscrewed a turn or so to relieve somewhat the pressure'by which the interleaved or overlapping portions of the two groups of laminations 18, 19 are pressed together, thereby decreasing somewhat the frictional forces holding the two groups of laminations in position. After this has been done the laminations are still held quite firmly in position but it is possible, by applying a sharp blow with a hard implement, to move one group of laminations relative to the other group. It will be understood that a certain amount of energy is required thus to relatively move the groups of laminations and due to the interleaved arrangement the amount of energy required to move one group of laminations relative to the other is substantial. Similarly the force required to overcome the static friction between the interleaved laminations is The ends of wire forming the relatively great even though the clamping pressure applied by the spring lock washers 48 is not very great. However, if the force is applied by impact of an implement having a hard Surface the distance that a group of laminations is moved is substantially a function of the kinetic energy present in the implement at the instant of impact. It is very easy for a workman to control the energy of impact by the velocity of the blow applied by the hammer or other implement used. Of course, this is true only if a hard surface implement is used to strike the blow since otherwise the energy of the impact will be absorbed mainly in deformation of the soft contacting surface.

For the purpose of increasing the air gap a screw driver 53, preferably having a metal or other hard shank and handle, is placed alternately against the heads of the screws 24 and the screw driver is tapped with a steel headed hammer 54 until the value of the inductance provided by the element is reduced to the desired value, in the manner as illustrated in Fig. 4. After the inductance has thus been adjusted the screws 44 may again be fully tightened so as to clamp the laminations securely in place. On the other hand, if it is desired to increase the value of the inductance, the screws 44 are partly loosened and the group of laminations 18 is struck at the center by the hammer 54 as indicated in Fig. 5, ifr

necessary by successive blows of progressively decreasing impact energy until the air gap is reduced sufficiently to cause the inductance to have the desired increased value. With very little practice it is easy to strike a blow with the impact velocity necessary for a large or a small change in the value of the inductance. By lightly tapping the core the inductance may be changed by extremely minute increments.

The inductance may be used advantageously as part of the tuning circuit of an oscillator, such as shown in Fig. 6, wherein a triode 60 is provided with a tuned circuit including the inductance 10 and either capacitor C62 or both the latter and a capacitor C64. The inductance 10 is provided with a tap 66 which is connected to the cathode of the triode 60 while one end of the inductance is connected to the grid of the triode through a blocking capacitor C68, the triode being provided with a grid return resistor R70 connected between the grid and cathode. ln the particular oscillator shown plate current is supplied from a +295 v. terminal of a power supply through either of switches 72, 73, respectively operable by keys 74 and 75. Closure of either switch 72 or 73 will result in the application of plate voltage to the plateof triede 60 through a suitable time delay mesh comprising resistor R76 and capacitor C77 in series, these elements being in parallel with resistor R78, and a capacitor C80 which connects the plate to ground. The

key 74, in addition Yto operating switch 72, operates a t 4 ductance 10 and ground. This oscillator is described in greater detail and is claimed in my copending application, Serial No. 96,107, filed May 28, 1949, now U. S. Patent No. 2,790,906.

By using the inductance described herein in an oscillator of the type shown in Fig. 6 fixed capacitors may be used in the tuning circuit and at no appreciable increase in cost over that of a nonadjustable inductance, the resonant point of the tuned circuit may be adjusted through a substantial range in large or very minute increments merely by appropriately governing the energy of impact of the implement by which oneV group of laminations is moved relative to the other. This is a very simple and effective manner of tuning the oscillator during the of its manufacture and inspection, and is also so :ple that it may be readily performed by the user of the instrument who has not had any experience in mechanical or electrical operations of this character.

While l have shown and described a particular embodiment of my invention, it will be apparent to those skilled in the art that numerous modifications and variations may be made in the form and construction thereof, without departing from the more fundamental principles of the invention. I therefore desire, by the following claim, to include within the scope of my invention all such similar and modified forms of the apparatus disclosed, by which substantially the results of the invention may be obtained by substantially the same or equivalent means.

I claim:

in combination, a mounting plate, an inductance element for use in tuning a circuit resonant at an audiofrequency comprising a coil, a core associated with the coil and having at least two groups of laminations providing an air gap therebetween, each group having portions of the laminations thereof interleaved with portions of the laminations of the other group, means for firmly switch 82 which effectively connects capacitor C64 in washer, and a screw extending through the washer, strap,

and mounting plate, and holding the inductance element on the plate, whereby one of the group of laminations Vmay be moved minute distances with respect to the other group hy sharply tapping one group with a hard,

implement in a direction to change the size of the air gap between the two groups of laminations.

References Cited in the le of this patent UNITED STATES PATENTS 2,445,088 Schilling July 13, 1948 2,460,145 Pfuntner et al. Ian. 25, 1949 2,541,797 Wagner Feb. 13, 1951 2,553,596 Mann May 22, 1951 2,554,782 Kiltie May 29, 1951- 

